1. Field of the Invention
The present invention relates to a photoconductive element for generation and/or detection of terahertz wave. The present invention relates also to an analysis apparatus, an imaging apparatus, and a communication apparatus employing the photoconductive element. The present invention relates also to a technique for adjusting the propagation state of a terahertz wave by selecting the electrode construction of the photoconductive element. In the present invention, the term “terahertz wave” denotes an electromagnetic wave occupying any portion of the frequency band ranging from 0.03 THz to 30 THz.
2. Description of the Related Art
The terahertz wave band covers characteristic absorption bands stemming from structures and states of various substances including biological molecules. By utilizing the characteristic absorption, inspection techniques are being studied for nondestructive analysis and identification of materials. These techniques are promising for safe imaging in place of X-ray radiography, and high-speed communication.
A photoconductive element is useful as an element for generation or detection of a terahertz wave. The photoconductive element has a semiconductor layer for generating carriers, and electrodes patterned with an interspace on the semiconductor layer for applying a voltage thereto. For improving the spatial coupling efficiency, an antenna structure may be provided on the parts of the electrodes with an antenna gap. On projection of a light beam to the antenna gap, carriers are generated in the semiconductor layer. The carriers are accelerated by application of a voltage between the electrodes to emit a terahertz wave from the photoconductive element.
The propagation characteristics of the terahertz wave in the photoconductive element depend generally on the state of carrier propagation as well as on the antenna structure. Herein the state of the carrier propagation includes the carrier behavior and the influence of the employed members such as absorption of phonon characteristic to the semiconductor substrate. For example, of the antenna for emission and reception of an electromagnetic wave, a bow-tie antenna has a broader band characteristics than a dipole antenna. However, for the photoconductive element to emit or receive terahertz wave, the dipole antenna is more suitable than the bow-tie antenna to obtain a terahertz wave in a broader frequency property range depending on the carrier propagation state. Appl. Optics 36, 7853 (1997) (hereinafter referred to as “Non-patent Document 1”) below discloses the above-mentioned matter. Japanese Patent Application Laid-Open No. 2006-010319 (hereinafter referred to as “Patent Document 1”) below discloses a method for controlling the propagation state of the terahertz wave generated or detected by the carrier by use of the bow-tie antenna in a photoconductive element.
The technique disclosed in Patent Document 1 is widely employed for controlling the propagation state of the terahertz wave generated or detected by the carriers by changing the structure of the antenna of the photoconductive element. Such a method of adjustment is effective, but requires an additional step for structure formation.